The invention relates to the head of a countersunk screw with an underside which has at least one support surface or edge, which support surface or edge forms part of a rotational surface around the screw axis and can cling to a correspondingly contoured countersunk surface of an add-on part, and at least one depression, which depression is delimited by a cutting edge at its trailing end, as seen in the screwing-in direction of the countersunk screw.
In the case of the known countersunk screw with a head of this type, the cutting edge delimits the leading end region of the support surface and is therefore located in the rotational surface. Should the screw be screwed into a prefabricated countersunk surface of a metallic add-on part, then the cutting edge can damage this countersunk surface, in particular coatings of the countersunk surface for improving the corrosion resistance, such as galvanic or inorganic metal layers or even plastic coatings (DE 33 34 212 A1).
The object of the invention is to construct the head of a countersunk screw of the type described at the beginning in such a manner that, on the one hand, when being screwed into soft material such as wood, the cutting edges can create their countersunk surface or a blocking action themselves and, on the other hand, when being screwed into metallic add-on parts with prefabricated countersinks, the cutting edges cannot damage the surface thereof.
Patent Claim 1 serves to achieve this object, according to which claim, a radially inwardly inclined advancing surface which abuts the support surface is arranged on a head of a countersunk screw according to the invention downstream of the said support surface as seen in the screwing-in direction, which advancing surface has the cutting surface at its leading free end, which cutting surface maintains an inward radial distance from the rotational surface over its length.
In the case of a head of a countersunk screw according to the invention, the cutting edge does not delimit the support surface, but rather an advancing surface, which preferably continuously adjoins the support surface in the screwing-in direction and is radially inwardly inclined.
As the countersunk surface coincides with the rotational surface in the screwed-in position of the screw, but does not run further radially inwards, the cutting edge keeps a radial distance from the countersunk surface over its entire length, so that the cutting edge cannot touch the countersunk surface or a coating applied thereto and therefore cannot damage the same. This distance can increase from the inside to the outside, as seen over the length of the cutting surface.
Nonetheless, it is also ensured in the case of the screw according to the invention that, in the case of use of the countersunk screw in soft material such as wood, the head creates the countersink itself by means of the cutting edge during screwing in of the countersunk screw.
It is preferred that each advancing surface continuously adjoins the support surface or edge assigned to it, preferably with a rounding.
Advantageous embodiments of the invention are described in the sub-claims.
In FIGS. 7 to 12, functionally identical parts have been assigned the same reference numbers as in FIGS. 1 to 6.